1.1 Field of the Invention
The present invention relates generally to the fields of sensor technology and molecular diagnostics. More particularly, it concerns the quantitative electrochemical detection of selected nucleic acid sequences in complex mixtures.
1.2 Description of Related Art
The determination of a specific DNA or RNA target nucleic acid sequence or segment present in air, food, water, environmental or clinical samples is of great significance in the medical, microbiology, food and water safety-testing, and environmental monitoring fields. The detection of the presence of a DNA or RNA sequence in a sample can rapidly and unambiguously identify genetic defects, oncogenic events and bacterial, viral or parasitic agents of concern.
Diagnosis of numerous infectious and inherited human diseases is possible with clinical assays that detect known DNA sequences characteristic of a particular disease (Molecular Diagnostics; 1993; Benn et al, 1987; Lowe, 1986). Unfortunately, few detection methods are suitable for routine diagnostic use either in the clinical laboratory or in the field setting. Many assays are not sufficiently rapid, inexpensive, simple or robust for routine application.
Recent advances in molecular diagnostics have focused on methods of detection at the genetic level. Since the advent of PCR™ technology, the ability to detect or identify point mutations, allelic variation, the presence of minute amounts of a pathogen, species, and from individual microscopic samples, to give a few examples, has been vastly improved. Yet even with the advances of PCR™ technology many limitations still exist which prevent diagnostic assays from being as versatile as desired or needed.
For example, one of the biggest problems with highly sensitive assays, such as PCR™ based assays, is contamination by extraneous air-borne organisms or by human contact. In general, if a molecular diagnostic assay is highly sensitive and can detect minute quantities of a selected or target nucleic acid segment, the sample to be assayed must be highly purified or at least not contain extraneous nucleic acid fragments which may be at least partially complementary to the target nucleic acid segment. Otherwise, false positive or ambiguous readings may result. Of course, obtaining highly purified samples is not cost ineffective due to time and labor involved. Strong technical expertise and well-equipped diagnostic laboratories are required in most cases. Thus in many instances where a highly sensitive assay is desirable, it is impractical, if not impossible, to perform such assays.
A variety of methods have been used to detect and quantitate polynucleotide sequences. Almost all of these begin with amplification of individual sequences or their transcripts by PCR. Some require separation by gel electrophoresis and/or laser detection. These needs Greatly complicate the development of good technology for scale-up, automation, and reduction of cost. Probe-array technologies are being developed by a number of companies—including Nanogen (San Diego, Calif.) and Affymetrix (Santa Clara, Calif.). The electrochemical methods described do not rely upon electrophoresis or laser technology, and can make use of DNA or PNA robes in arrays.
In cases where it is desirable to detect more than one target nucleic acid species in a sample, or the sample is highly complex, highly sensitive assays must be customized to detect the desired targets. Although not entirely understood, it is well known that many highly sensitive assays suffer from undue interference caused by background sample material. In fact, the time and labor required to develop some assays is so great that the assays are not useful and less sensitive means of analysis are employed.
Diagnostic assays that do not require high purity samples are less sensitive and therefore cannot detect minute amounts of target. Thus larger or more concentrated samples must be obtained. In some cases, larger sample quantities are not available or are too costly.
Probe assays, including oligonucleotide-probe and gene-probe assays, have been developed recently in an attempt to take advantage of the ability to detect selected DNA or RNA sequences with high sensitivity and replace conventional detection methods. However, these techniques still tend to be labor-intensive and often require significant technical training and expertise. Further, highly sensitive gene-probe assays require specialized equipment and are generally not compatible with field settings. Those that can be used in a field setting are limited to determining the presence or absence of a target nucleic acid fragment and do not provide quantitative information. The utility of gene-probe assays for environmental monitoring and other uses outside of a laboratory setting is limited.
Moreover, gene-probe assays may use a label that is either toxic or requires substantial expertise and labor to use. Radiolabeling is one of the most commonly used techniques because of the high sensitivity of radiolabels. But the use of radiolabeled probes is expensive and requires complex, time consuming, sample preparation and analysis and special disposal. Alternatives to radioactivity for labeling probes include chemiluminescence, fluorimetric and colorimetric labels (Kricka, 1993) but each alternative has distinct disadvantages. Colorimetry is relatively insensitive and has limited utility where minute amounts of sample can be obtained. Samples must also be optically transparent. Fluorimetry requires relatively sophisticated equipment and procedures not readily adapted to routine use. Chemiluminescence, although versatile and sensitive when used for Southern blots, northern blots, colony/plaques lift, DNA foot-printing and nucleic acid sequencing, is expensive, and is not well-adapted for routine analysis in the clinical laboratory.
Another limitation to the versatility of oligonucleotide-probe assays is that virtually all current oligonucleotide-probes are designed as heterogeneous assays, i.e., a solid phase support is used to immobilize the target nucleic acid so that free, non-hybridized probe can be removed by washing. Complex procedures and long incubation times (one to several days) are usually required which makes these assays difficult to incorporate into the simple and rapid formats that are desirable for clinical applications or on-site analysis (Molecular Diagnostics, 1993).
Various techniques exist for the detection of differential gene expression into closely matched cell populations. These include subtractive cloning (Sagerstrom et al., 1997, Differential Display (Liang and Pardee, 1992), serial analysis of gene expression (SAGE) (Velculescu et al., 1995), expressed sequence tags (ESTs) (Adams et al. 1991). While all of the above-mentioned approaches have yielded significant results most have drawbacks. Subtractive cloning, SAGE and ESTs tend to be labor intensive and costly and therefore not suitable for use on a routine basis. For example SAGE and ESTs entail the use of hundreds or thousands of DNA sequencing reactions. Recently the feasibility of hybridization based technology as well as its superiority to other screening process has been shown. The human genome project has identified new genes and unique ESTs at a rapid rate. As of Nov. 30, 1998 17,583 human genes or complete coding sequences had been identified and 52,277 unique ESTs had been cataloged. DNA microarray or gene chip technology has catalyzed further advances. The technology involves the positioning of highly condensed and ordered arrays of DNA probes on glass slides or nylon membranes. Up to 50,000 DNA fragments, each representing an individual gene can be placed on a single glass slide and up to 5,000 placed on a nylon membrane. The resulting microarrays can then be used to examine presence and levels of gene expression.
Alternatives to gene-probe and other assay methods of detecting nucleic acid sequences have employed electrochemical biosensors that employ intercalators and discriminate between immobilized single-stranded and double-stranded DNA (Hashimoto et al., 1994; Millan et al., 1994; Millan and Mikkelsen, 1993). While such biosensors are capable of detecting a known target DNA sequence, they are handicapped by the fact that the electrode must be cleaned between each use. The procedures used to strip away the hybridized target DNA from the electrode surface are not suitable for widespread screening applications, such as clinical diagnostics where labor and expense must be kept minimal and speed is essential, or in settings outside of the laboratory such as field testing.
DNA diagnostics have recently achieved importance because of the advances in molecular biology that have highlighted the importance of gene mutations and hereditary diseases. Many studies have focused on the breast cancer susceptibility gene BRCA1 which is estimated to account for a large fraction of hereditary breast cancer and the majority of familial breast/ovarian cancer. Over 111 unique BRCA1 mutations distributed throughout the gene have been described (Shatkuck-Eldens et al., 1995: Breast Cancer Information Core Database). Many methods have been used to screen for BRCA1 mutations. Almost all of these based on amplification of individual exons or their transcripts by PCR™ (Nollau and Wagener, 1997). All require separation by gel-electrophoresis. The need for electrophoresis greatly complicates scale-up automation and streamlining of procedures. A method that does not require electrophoresis (Affymetrix, Santa Clara, Calif.) is epifluorescence confocal scanning microscopy that utilizes high-density oligonucleotide arrays of over 96,000 oligonucleotides to scan 3,450 bases of exon 11 of BRCA1 (Hacia et al., 1996). Although it is powerful technique it is expensive and not readily available to most laboratories.
There are many human diseases associated with known gene alterations. These diseases include cystic fibrosis, muscular dystrophy, sickle cell anemia, phenylketonuria, thalassemia, hemophelia, α1-antitrypsin deficiency and lipoprotein metabolism disorders (Ben et al., 1987; Lowe, 1986; Landegren et al., 1988; Young et al., 1989; Kricka, 1993). Quantitative analysis of human genes is also useful for analysis of amplified oncogenes (Altitalo, 1987) and in a measurement of gene expression levels in tumors (Slamon et al., 1989). Genetic aspects of human diseases contributed by factors such as BRCA1, BRCA2 and p53 mutations all of which confer high cancer risk would benefit from gene analysis as would mutational changes in genes caused by chemical radiation.
Breast cancer is considered to have a hereditary cancer risk component. Among women who have a blood relative with the disease the risk of developing breast cancer is 1 in 5. Two mutated genes increase a woman's chances of developing breast cancer and genetic tests for detecting women at greatest risk have been used clinically.
Several genes in have been associated with the pre-disposition to cancer. These include BRCA1, p53, RBI and APC which are just a few of the more than 20 genes identified that are associated with the pre-disposition to cancer (Fearon, 1997; Ponder, 1997). The BRCA1 gene and its expressed protein as it relates to diagnosis and potential treatment of disease, has utility in diagnosis and potential disease treatments.
Clearly, there is a need for improved detection of nucleic acid sequences. Unfortunately, few assays are currently available for routine monitoring and/or diagnostic use because of the expense, complexity and/or physical limitations which prevent their use outside of a well-equipped laboratory. As discussed, the few existing assays that have limited applications and do not meet the diverse needs of clinical diagnostics and field testing.